Fig. 12.16 ). A single action potential releases sufficient Ca++ to cause a twitch contraction. However, the duration of this contraction is very short because Ca++ is very rapidly pumped back into the SR. If the muscle is stimulated a second time before it is fully relaxed, the force of contraction increases (see
Fig. 12.16 , middle). Thus twitch forces are amplified as stimulus frequency increases. At a high level of stimulation, intracellular [Ca++] increases and is maintained throughout the period of stimulation (see
Fig. 12.16 , right), and the amount of force developed greatly exceeds that observed during a twitch. The response is termed tetany. At intermediate stimulus frequency, intracellular [Ca++] returns to baseline just before the next stimulus. However, there is a gradual rise in force (see
Fig. 12.16 , middle). This phenomenon is termed incomplete tetany. In both cases, the increased frequency of stimulation is said to produce a fusion of twitches.
The low force generation during a twitch, in comparison with that during tetany, may be due to the presence of a series elastic component in the muscle. Specifically, when the muscle is stretched a small amount shortly after initiation of the action potential, the muscle generates a twitch force that approximates the maximal tetanic force. This result, coupled with the observation that the size of the intracellular Ca++ transient during a twitch contraction is comparable with that during tetany, suggests that enough Ca++ is released into the myoplasm during a twitch to allow the actin-myosin interactions to produce maximal tension. However, the duration of the intracellular Ca++ transient during a twitch is sufficiently short that the contractile elements may not have enough time to fully stretch the series elastic components in the fiber and muscle. As a result, the measured tension is submaximal.
An increase in the duration of the intracellular Ca++ transient, as occurs with tetany, provides the muscle with sufficient time to completely stretch the series elastic component and thereby results in expression of the full contractile force of the actin-myosin interactions (i.e., maximal tension). Partial stretching of the series elastic component (as might be expected during a single twitch), followed •Fig. 12.17. Slow-Twitch Muscles Exhibit Tetany at a Lower Stimulation Frequency Than Do Fast-Twitch Muscles. A, Fast-twitchmotorunitinthegastrocnemiusmuscle.B, Slow-twitchmotorunitinthegastrocnemiusmuscle.C, Slow-twitchmuscleunitinthesoleusmuscle.Themotorunitswerestimulatedatthefrequenciesindicatedontheleft.Thecalibrationbarfortension(ingrams)generatedduringconcentrationisindicatedbythevertical brackets underthecurves.Notethelargeforcegeneratedbythefast-twitchmotorunit(A).(FromMontcastleV[ed].Medical Physiology. 12thed.St.Louis:Mosby:1974.) by restimulation of the muscle before complete relaxation, however, would be expected to yield an intermediate level of tension, similar to that seen with incomplete tetany. The location of the series elastic component in skeletal muscle is not known. One potential source is the myosin molecule itself. In addition, it is likely that there are other sources of the series elastic component, such as the connective tissue and titin.
The stimulus frequency needed to produce tetany depends on whether the motor unit consists of slow or fast fibers (
Fig. 12.17 ). Slow fibers can be tetanized at lower frequencies than can fast fibers. The ability of slow-twitch muscle to tetanize at lower stimulation frequencies reflects, at least in part, the longer duration of contraction seen in slow fibers. As also illustrated in
Fig. 12.17 , fast fibers develop a larger maximal force than slow fibers do because fast fibers are larger in diameter than slow fibers and there are more fibers in fast motor units than in slow motor units.
Modulation of Force by Reflex Arcs